Aging Aircraft Conference Focuses on Electrical Systems

Air Safety Week, Sept 27, 1999

Added to this combination of mechanical and chemical influences on wire age is the complicating contribution of polymer breakdown. Wire insulation with a molecular chain of some 400 units when new can break down, over time, into chains only some 20-40 units long. The phenomenon is known as chain scission. Add stress, vibration and sharp curvatures to scission, and the insulation is even more likely to fail.

The economics of wire maintenance

According to Eaton, the Navy will spend about $21 million in unscheduled wire maintenance in fiscal 1999, a figure that works out to about $52 per flight hour, and about 5 percent of all flying hour costs. Eaton compared this figure to the 737-200 fleet, some 400 of which are in U.S. registry. Based on officially reported maintenance data, the airlines are spending some $21 per flight hour on wire maintenance, or about $66,000 per aircraft per year.

The difference may be attributable to the more severe operating environment for Navy aircraft, but Eaton raised a different question: Maybe the Navy is spending too much but, then again, maybe the airlines are not spending enough.

What the Navy has found, though, is that prevention is worth the effort. When the Navy replaced severely aged wire, the rate of inflight electrical fires plummeted nearly 90 percent in five years. "Where no corrective actions were taken, further failures resulted," Eaton intoned. In addition, he pointed out, the least costly unscheduled wire repair, some $320, is twice the cost of available non-destructive evaluation (NDE) techniques.

The trouble-shooting challenge

When the roof blew off the Aloha jetliner, the industry was faced with a whole new troubleshooting challenge. Visual inspections had not caught the pernicious spread of cracking between the rivets on the overhead lap joint of the Aloha aircraft. Indeed, the need to identify fatigue cracking and damage from corrosion spawned the development of a whole range of new technologies. Today, an automated, robot-like system can be seen quietly marching along a track attached to the airplane's outer skin with suction cups. The scanner flicks back and forth across the lap joint, displaying cracks and corrosion on a nearby computer screen. Boeing's MAUS IV (Mobile Automated Scanner) represents an example of the latest in such technology for assessing structural integrity.

For aircraft electrical systems, the possible solutions outlined at last week's conference drew from sources as varied as household to space technology.

Gary Scott, an engineer with the Square D Co. in Cedar Rapids, Iowa, described how his company is developing arc-fault circuit interrupters (AFCI) for household use, and for their possible application to airliners. In 1994, some 370 Americans were killed from electrical distribution system fires in their homes - a rate of about one death daily. The situation prompted the U.S. 1999 National Electric Code to include a requirement for the use of arc-fault detection breakers in new bedroom circuits beginning January 1, 2002. The mandate stems from the fact that current circuit breakers are not protecting against intermittent faults. The parallel to the airline industry is obvious. The company has developed a prototype breaker to meet this new home-construction market. According to Scott, the challenge is to refine the technology for aircraft use, and to shrink the breaker from its home-use size (about that of a small paperback book) to the matchbook size AFCI that would be needed meet the space constraints of aircraft electrical panels.